Here's a simple guideline for how much water to drink when you exercise: drink only when you feel thirsty. Science Daily reports that many runners and other athletes may be drinking too many fluids when experts recommend drinking only when thirsty.

A survey found that over a third of runners were drinking according to a preset schedule or for weight maintenance, while 9 percent were drinking as much as possible. Nearly a third incorrectly thought they needed extra salt and more than half were drinking sports drinks for the electrolytes to prevent low blood sodium.

But it's actually "drinking too much during exercise [that] can dilute the sodium content of blood to abnormally low levels," which could potentially lead to a fatal condition called exercise-associated hyponatremia, the article says.

"The safest known way to hydrate during endurance exercise" is to only drink when thirsty, Loyola sports medicine doctor James Winger advised. Photo by lululemon athletica.